The Facebook chief will be tested as he appears before Congress about privacy issues

A year ago, Mark Zuckerberg was preparing to deliver the commencement speech at Harvard University. As well as a personal milestone, it was the kind of carefully choreographed, profoundly upbeat event at which he excels.

This week, Mr. Zuckerberg must deliver an entirely different type of performance. In two days of testimony before Senate and House committees, the sometimes-defiant Facebook Inc. chief executive must prove to skeptical lawmakers that the tech behemoth he created takes privacy issues seriously. It also is...

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